Blood loss meter



May 4, 1965 J. VAN DEN BERG BLOOD LOSS METER Filed Dec.

Tempered are compcnsaiinq resistor llnl-lll'lll United States Patent3,182,252 BLOOD LOSS METER Janwillem van den Berg, Petrus Campersingel151a, Groningen, Netherlands Filed Dec. 16, 1960, Ser. No. 76,294 4Claims. (Cl. 324-30) The invention relates to a device for measuring theamount of blood lost by a patient during operations and the like, andmore in particular to a conductometer specially adapted to blood lossmeasurements.

Up to now, the blood loss of a patient during operations and the likewas generally determined by Weighing .means whereby the blood loss of apatient may be directly determined by conductometry.

A further object of the invention is to provide a conductometerespecially adapted to blood loss measurements, and having a scalebearing a linear calibration in volume parts of blood.

A still further object of the invention is to provide a conductometerfor blood loss measurements which may be easily adjusted to the electricconductivity of the blood of an individual patient.

Another object of the invention is to provide a conductometer for bloodloss measurements which may be calibrated in accordance with theelectric conductivity of the blood of an individual patient with the aidof a relatively small amount of blood.

Still another object of the invention is to provide a conductometer forblood loss measurements of which the operation is independent oftemperature variations and of polarization phenomena.

Still another object .of the invention is to provide a conductometer forblood loss measurements wherein the Washing liquid may be easilyadjusted to the desired conductivity with the aid of the blood lossindicator.

In the apparatus according to the invention, the blood loss of a patientis determined with the aid of a washing liquid having a predeterminedelectric conductivity. A predetermined amount of this Washing liquid,for instance 50 litres, is placed in a measuring vessel provided withapowerful agitating member. The washing liquid may be water, adjusted tothe desired conductivity by the addition of an electrolyte. The.dressings containing the blood lost by the patient are thrown into thewashing liquid immediately after use; by the action of the agitatingdevice, they are rinsed so effectively that the blood is substantiallyimmediately dispersed throughout the washing liquid. This causes theelectric conductivity of the washing liquid to be changed by an amountdependent on the amount of blood dispersed through the washing liquid.Thus, the blood loss of the patient may be determined by measuring theconductivity of the liquid mixture in the vessel.

The measurement of the conductivity of the liquid mixture is performedby means of a conductometer comprising an A.C. bridge circuit, of whichone of the arms contains the path between two electrodes immersed in thevessel, and which is kept in equilibrium by supplying the voltagebetween two opposite junctions through an amplifier to a servo-motorcontrolling a variable resistor, inserted in one of the other bridgearms and coupled with an indicator moving along a scale.

It is extremely important that the resistance value of the variableresistor is a linear function of the blood loss in volume parts. Notonly does this condition provide for a clearly readable scale, but inaddition, the fact must be taken into account that there exist largedifferences between the conductivities of the blood of various patients.The said linear function makes it possible to calibrate the device inaccordance with the conductivity of the blood of the patient in a simpleand exact manner.

According to a main feature of the invention, this linear dependency isobtained by inserting the variable resistor in a bridge arm which isdiametrically opposed to the bridge arm containing the electrodes, andwhich comprises the series connection of a first and a second resistorof which the resistance values are in a ratio of (g g :g wherein g isthe conductivity of the washing liquid and g the conductivity of theblood of the patient, the variable resistor being connected in parallel'be immersed. Thus, only a few cubic centimeters of blood have to betaken from the patient for calibration purposes.

The conductivity g of the washing liquid prior to the introduction ofthe blood is selected in such manner as to be higher than the maximumconductivity of water occurring in practice, so that the desired valuemay always be obtained by the addition of an electrolyte. The adjustmentis preferably performed in the measuring vessel, so that use may be madeof the blood loss indicator for checking the conductivity. Uponintroduction of the untreated water into the measuring vessel, theindicator coupled with the variable resistor will take a position to theleft of the zero point of the scale, and the zero point .will begradually reached by the addition of the electrolyte. In order tosimplify this operation, it is preferred to connect a resistor inparallel with the path between the electrodes, whereby the zero point isdisplaced to the right. This parallel resistor is so dimensioned thatthe indicator is placed opposite to a marking on the scale spaced withrespect to the Zero point when the conductivity of the washing liquidhas reached the desired value by the addition of the electrolyte.

The invention will be more fully explained by reference to theaccompanying drawing showing a preferred embodiment thereof.

The drawing illustrates a circuit diagram of a conductometer for bloodloss measurements according to the invention.

The device shown in the drawing comprises a measuring vessel V, providedwith an agitating member RW and filled to the level N with water whichhas been adjusted to .a predetermined electric conductivity by theaddition of an electrolyte. The measuring vessel may contain, forinstance, litres of this washing liquid. In the case of operationsperformed on children, wherein the blood loss is much less, a smallerquantity of the washing liquid may be used, for instance 15 litres. Alldressings by which the blood of the patient has been absorbed are throwninto the measuring vessel immediately after use.

The construction of the measuring vessel is not bound to strictrequirements, provided that the agitating memher is so powerful that alldressings thrown into the measuring vessel are rinsed practicallyimmediately. Good results have been obtained in practice with domesticWashing machines having usual kind.

The measuring vessel contains a measuring cell (not shown), whichcomprises, inter alia, the electrodes E and E The path between theseelectrodes is inserted in a,

a pulsato-r or agitator of the if) first arm of a bridge circuit fedwith A.C. current through terminals K and K For this purpose, use may bemade of the mains current of 60 cycles per second, or of a speciallygenerated A.C. current having a higher frequency for instance of 1000cycles per second.

A second bridge arm, which is diametrically opposed to the first armcontaining the measuring electrodes, comprises a first resistor R asecond resistor R in series with resistor R and a Variable thirdresistor R in parallel with resistor R The third bridge arm contains afourth resistor R Resistors R and R are simultaneously adjustable bymeans of an adjusting member I, in order to calibrate the device inaccordance with the conductivity of the blood of the patient to betreated. Resistor R is controlled by a servo-motor SM fed through anamplifier VS by the voltage between opposite junctions of the bridgecircuit. Thus, resistor R is always adjusted in such manner that thebridge circuit is in equlibrium. Resistor R is coupled with an indicatormoving along a scale SC calibrated in cm. of blood. Prefer-ably, scaleSC comprises two calibrations, one running from to 3000 0111. of bloodin 50 litres of washing liquid, and the other from 0 to 1000 cm. ofblood in 50 litres of washing liquid. The second calibration may also beused during operations on children for indicating 0 to 300 cm? of bloodin 15 litres of washing liquid. The two calibrations may .be obtained,for instance, by using two indicators coupled with the adjusting memberof resistor K, through different transmissions.

The fourth bridge arm contains a temperature compensating resistorassembly consisting of a fifth resistor R of which the resistance isdependent on temperature, a sixth resistor R in series with resistor Rand independent of temperature, and a seventh resistor R in parallelwith the series connection of resistors R and R and independent oftemperature. The resistors R R and R have been chosen in such mannerthat the assembly has the same resistance-temperature co-efiicient asthe washing liquid. The assembly is enclosed in the measuring celltogether with electrodes E and E so that it is always at the sametemperature as the Washing liquid. By these means, the blood lossmeasurement is rendered independent of the temperature of the washingliquid. The resulting resistance value of the assembly will be indicatedby the symbol R hereinafter. A variable condenser C is connected inparallel with said assembly in order to compensate for the phasedisplacement caused by polarization of the electrodes E and E An eighthresistor R may be connected in parallel with the paths betweenelectrodes E and E by means of a switch S, for purposes to be describedhereinafter.

In order to obtain a linear dependency between the resistance value ofresistor R and the amount of blood in cm. introduced into the measuringvessel, resistors R and R have been selected in such manner that theirresistance ratio is as (g g :g wherein g is the conductivity of thewashing liquid, and g the conductivity of the blood of the patient to betreated. This may be exnlained as follows:

Assume that the measuring vessel V contains A litres of washing liquid,and that x litres of blood are added. The resulting conductivity isapproximately equal to so that the resistance measured betweenelectrodes E and E is equal to k(A+x) 'i'Re)=R:R3 (1) 4 By introducingthe condition that R =0 for x=0, Equation 1 becomes:

By eliminating R R from Equations 1 and 2 and solving for R one finds:

Thus, the resistance R; is found to be linearly dependent on the amountof blood 2:.

In order to be able to use the same scale under all circumstances, Rmust be constant according to Equation 5. Thus, Equation 4 may only besatisfied for all values g by making R variable. It appears fromEquation 2 that one of the resistors R and R must also be variable inthis case. Since R is preferably left constant in view of thetemperature compensation, resistors R and R are simultaneouslyadjustable in the embodiment as shown in the drawing. In practice, theadjustment of resistors R and R may be performed by adding constantresistors in a plurality of stages; for this purpose, the adjustingmember I is constructed as a tandem switch.

The calibration of the apparatus according to the conductivity of theblood of the patient to be treated is erformed by introducing apredetermined quantity, for instance 50 cm. of the washing liquid,adjusted to the desired conductivity into vessel Y from vessel V, and byadding a suitable small quantity of blood, for instance 1 cm. taken fromthe patient to be treated. After that the measuring cell is placed intovessel Y. Since resistor R is adjusted, according to Equation 5, to thevalue R which is proportional to the ratio x/A between the amounts ofblood and washing liquid, the scale SC Will have to indicate an amountof 1000 cm. of blood in 50 litres of washing liquid in the present case.Resistors R and R are now adjusted, by means of the adjusting member I,in such manner that this indication is obtained.

In order to adjust the washing liquid to the desired conductivity,vessel V is filled with water, after which switch S is closed, wherebythe zero point of the scale is displaced to the right. After that, asuitable salt is added to vessel V until a marking on scale SC has beenreached.

Although the invention has been described hereinbefore by reference to aspecific embodiment thereof, it is to be understood that the inventionis not restricted to this embodiment, which may be modified in variousways within the scope of the invention as set forth in the appendedclaims.

What is claimed is:

1. A blood loss meter comprising a measuring vessel adapted to receive awashing liquid of known electric conductivity, an agitator in saidmeasuring vessel adapted for mixing the blood adsorbed by dressings withsaid Washing liquid, an AC. bridge circuit including four junctions andfour arms connected between the said junctions, an alternating currentsource connected between two diametrically opposed junctions, a pair ofelectrodes in said measuring vessel and connected in a first arm of saidbridge circuit, a first resistor, a second resistor in series with saidfirst resistor and connected together with the same in a second arm ofsaid bridge circuit diametrically opposed to said first arm, a variablethird resistor in parallel with said first resistor, a fourth resistorconnected in a third arm of said bridge circuit, a resistive impedanceconnected in the fourth arm of said bridge circuit, an amplifier havingan input connected with the remaining diametrically opposed junctions ofsaid bridge circuit, a servo-motor coupled to said amplifier andcontrolling said third resistor to keep the bridge circuit inequilibrium, an indicator mechanically coupled with said third resistor,a scale calibrated in volume parts of blood and positioned opposite tosaid indicator, means for adjusting the ratio between the resistancevalues of said first and said second resistor, so that this ratio may beadjusted, for a given patient, to (g g ):g wherein g is the electricconductivity of the washing liquid and g the electric conductivity ofthe blood of the patient, and means operated simultaneously with saidratio adjusting means for adjusting the resistance value of an arm ofsaid bridge circuit adjacent to said first arm in such manner that theequilibrium of the bridge circuit is maintained.

2. A blood loss meter as claimed in claim 1 further comprising a fifthresistor, and means for connecting said fifth resistor in parallel withthe path between the said electrodes in order to shift the zero point ofsaid scale, so that the electric conductivity of the washing liquid maybe adjusted to the desired value with the aid of said indicator.

3. A meter as claimed in claim 1 wherein said resistive impedancecomprises a fifth resistor dependent on temperature, a sixth resistor inseries with said fifth re- References Cited by the Examiner UNITEDSTATES PATENTS 2,224,382 12/40 Douty 324 2,899,636 8/59 Rubricius 32430OTHER REFERENCES Rosenthal et a1.: Measurements of the ElectricResistance of Human blood September 1948, vol. 33, No. 9, The Journal ofLaboratory and Clinical Medicine (RBI J 6), pp. 1117-1120 of pp.1110-1122 relied on.

WALTER L. CARLSON, Primary Examiner.

SAMUEL BERNSTEIN, FREDERICK M. STRADER,

Examiners.

1. A BLOOD LOSS METER COMPRISING A MEASURING VESSEL ADAPTED TO RECEIVE AWASHING LIQUID OF KNOWN ELECTRIC CONDUCTIVITY, AN AGITATOR IN SAIDMEASURING VESSEL ADAPTED FOR MIXING THE BLOOD ADSORBED BY DRESSINGS WITHSAID WASHING LIQUID, AN A.C. BRIDGE CIRCUIT INCLUDING FOUR JUNCTIONS ANDFOUR ARMS CONNECTED BETWEEN THE SAID JUNCTIONS, AN ALTERNATING CURRENTSOURCE CONNECTED BETWEEN TWO DIAMETRICALLY OPPOSED JUNCTIONS, A PAIR OFELECTRODES IN SAID MEASURING VESSEL AND CONNECTED IN A FIRST ARM OF SAIDBRIDGE CIRCUIT, A FIRST RESISTOR, A SECOND RESISTOR IN SERIES WITH SAIDFIRST RESISTOR AND CONNECTED TOIGETHER WITH THE SAME IN A SECOND ARM OFSAID BRIDGE CIRCUIT DIAMETRICALLY OPPOSED TO SAID FIRST ARM, A VARIABLETHIRD RESISTOR IN PARALLEL WITH SAID FIRST RESISTOR, A FOURTH RESISTORCONNECTED IN A THIRD ARM OF SAID BRIDGE CIRCUIT, A RESISTIVE IMPEDANCECONNECTED IN THE FOURTH ARM OF SAID BRIDGE CIRCUIT, AN AMPLIFIER HAVINGAN INPUT CONNECTED WITH THE REMAINING DIAMETRICALLY OPPOSED JUNCTIONS OFSAID BRIDGE CIRCUIT, A SERVO-MOTOR COUPLED TO SAID AMPLIFIER ANDCONTROLLING SAID THIRD RESISTOR TO KEEP THE BRIDGE CIRCUIT INEQUILIBRIUM, AN INDICATOR MECHANICALLY COUPLED WITH SAID THIRD RESISTOR,A SCALE CALIBRATED IN VOLUME PARTS OF BLOOD AND POSITIONED OPPOSITE TOSAID INDICATOR, MEANS FOR ADJUSTING THE RATIO BETWEEN THE RESISTANCEVALUES OF SAID FIRST AND SAID SECOND RESISTOR, SO THAT THIS RATIO MAY BEADJUSTED, FOR A GIVEN PATIENT, TO (G2-G1):G1, WHEREIN G1 IS THE ELECTRICCONDUCTIVITY OF THE